The performance of organic photovoltaic (OPV) devices is most frequently characterized by the power conversion efficiency, ηp, which indicates the percentage of the radiant energy incident on the solar cell that is converted to electrical energy, and is determined by measuring the open-circuit voltage (Voc), short-circuit current (Isc), and fill factor from current-voltage plots. Although ηp serves as a convenient benchmark for comparing photovoltaic devices, it does not reveal information about local photocurrent spatial variations within these devices. Localized efficiency variations might be expected in bulk-heterojunction (BHJ) OPVs from defects as well as from the interpenetrating phase-separated nature of the microstructure, evident in morphological studies by atomic force microscopy, transmission electron microscopy, scanning electron microscopy, and scanning transmission X-ray microscopy.
Correlations between electrical properties and morphology in bulk heterojunction OPV films have previously been demonstrated with scanning probe techniques. For example, a morphology-work function relationship in poly(2-methoxy-5-(3′,7′-dimethyloctyloxy))-p-phenylene vinylene: [6,6]-phenyl-C61-butyric acid methyl ester (MDMO-PPV:PCBM) films was demonstrated by Kelvin probe force microscopy (KPFM). Similarly, a correspondence between morphology and photocurrent was established in polyfluorene films by near-field scanning photocurrent microscopy (NSPM). However, NSPM lateral resolution is limited to ˜200 nm by the tip aperture.
Atomic force microscopy is described generally in U.S. Pat. No. 6,642,517, the entirety of which—and, in particular, FIGS. 1-2, 4 and 6-7 and corresponding descriptions thereof and the references cited therein—is incorporated herein by reference. More specifically, conductive atomic force microscopy (cAFM) has recently proven to be an effective method for probing current flow and resistivity variations with nanometer scale spatial resolution in gold nanowires, silicon field effect transistors, individual organic molecules, conducting polymer blends, and emissive polymers. See, respectively: M. C. Hersam, A. C. F. Hoole, S. J. O'Shea, and M. E. Welland, Appl. Phys. Lett. 72, 915 (1998); P. De Wolf, W. Vandervorst, H. Smith, and N. Khalil, J. Vac. Sci. Technol. B 18, 540 (2000); A. M. Rawlett, T. J. Hopson, L. A. Nagahara, R. K. Tsui, G. K. Ramachandran, and S. M. Lindsay, Appl. Phys. Lett. 81, 3043 (2002); J. Planes, F. Houzé, P. Chrétien, and O. Schneegans, Appl. Phys. Lett. 79, 2993 (2001); and H.-N. Lin, H.-L. Lin, S.-S. Wang, L.-S. Yu, G.-Y. Perng, S.-A. Chen, and S.-H. Chen, Appl. Phys. Lett. 81, 2572 (2002).
Since the cAFM tip is used locally, cAFM can directly correlate optoelectronic stimulation with nanometer scale spatial resolution. With appropriate collection optics and photon detectors, the resulting photocurrent can be spatially correlated with the cAFM tip position, thus enabling nanometer scale photocurrent mapping. cAFM is described generally in U.S. Pat. No. 5,874,734, the entirety of which is incorporated hereby by reference. Thus, cAFM and analogous scanning tunneling microscopy measurements have been used to spatially map current in a variety of organic materials. However, in these studies, the conductive tip was brought directly into contact with the organic material. Again, while such direct electrical contact with the materials is sufficient to induce current, a point contact of this type is inevitably different from the evaporated electrical contacts fabricated in actual photovoltaic devices.
Recently, two scanning probe techniques having sub-100 nm lateral spatial resolution and calibrated light sources were reported: 1) Time-resolved electrostatic force microscopy (trEFM) was used to simultaneously measure localized photoinduced charging rates and topography in a polyfluorene film, 2) Photoconductive AFM (pcAFM) employed a laser to illuminate a BHJ film while measuring topography and photocurrent with a conductive platinum-coated probe. Again, while both techniques provide quantitative correlations between electrical properties and morphology, they characterize photovoltaic films rather than functioning photovoltaic devices.